1. Field of the Invention
This invention relates to a positron lifetime spectrometer based on a DC positron beam. More particularly, it relates to a positron lifetime spectrometer having a wide valid time range that results from a uniquely straightforward spectrometer design and an improved time resolution brought about by simultaneously accelerating both the primary positrons and the secondary electrons produced by the spectrometer.
2. Background Information
As materials are engineered and manipulated at nanometer scale, defects in those materials become very important because they can drastically alter the material properties and device functionality. Conventional techniques such as cross section transmission electron microscopy (XTEM) and neutron scattering are limited by their selectivity and sensitivity to micro open-volume defects. Positron lifetime spectroscopy (PLS), on the other hand, is fundamentally sensitive to open-volume defects because positrons are preferentially trapped at defect sites. In addition, positron spectroscopy is a nondestructive technique.
When positrons enter a solid, they are trapped in vacancies, vacancy clusters and voids because these sites provide local minima in the potential energy. The trapped positrons annihilate with the electrons of surrounding atoms, generating gamma (xcex3) radiation that is used to signature defects. By varying the energy of the incident positrons, the characterization of defects as a function of depth in the material can be carried out.
In one current method, positron lifetime spectroscopy based on an isotope positron source detects size and concentration of defects in bulk materials. Isotope-based PLS, however, is a non-beam type of PLS that cannot characterize defects in thin films or in their substrate interfaces because positrons generated by an isotope yield a broad energy distribution varying from 0 to 500 keV.
In recent years, positron lifetime spectroscopy in conjunction with either a variable energy pulsed positron beam or a variable energy DC positron beam has been found to be increasingly useful for characterizing defects in thin films and their substrate interfaces as semiconductor and optical devices are made in nanometer scale.
The measures of performance of a positron lifetime spectrometer include 1) the time range over which a sum of exponential decay functions are fitted to the lifetime spectrum without interference from spurious structures, i.e., xe2x80x9cghostxe2x80x9d peaks; 2) the time resolution, defined as the full width of half-maximum (FWHM) of a single time peak; and 3) the signal-to-noise ratio.
Schodlbauer et al of the Institut fur Nukleare Festkorperphysik of Germany has conducted positron lifetime spectroscopy using a pulsed positron beam. That spectrometer chops a DC positron beam into pulses, then bunches the pulses into narrow pulses. An advantage of the spectrometer system is its ability to generate very narrow pulses. A disadvantage is that the system requires the construction of a buncher, which is very expensive. Another disadvantage is the loss of many positrons during the chopping and bunching processes.
Suzuki et al of the Electrotechnical Laboratory in Japan has a different pulsed beam positron lifetime spectrometer. It employs a longitudinal rather than transversal chopper, and it also utilizes a positron buncher. The spectrometer reduces positron loss during the chopping process. However, its disadvantages are the same as those of the Munich spectrometer.
Closer to the present invention is a positron lifetime spectrometer originated by Lynn et al at Brookhaven National Laboratory, and revised by Szpala. Lynn""s original spectrometer is based on a DC positron beam, and it uses the secondary electrons generated by the primary positrons as a time signal. The revised spectrometer relies on an additional electrode, called a retarding grid, to accelerate the electrons. However, the electrical potential applied to the retarding grid (+600 V) decelerates the incoming positrons. Therefore, the potential cannot be too high ( less than 600 V) or it interferes with the incoming positrons. This constraint limits the reduction of the time spread induced by the electron energy distribution. The spectrometer also uses an ExB field for separating the electron flight path from that of the primary positrons.
The valid time range of the Szpala revised spectrometer is 0-3 nsec. After that, there are some xe2x80x9cghostxe2x80x9d structures, possibly due to the contributions from back-scattered positrons annihilating in the ExB separator. The spectrometer has a time resolution of 475 psec. The signal-to-noise ratio of the Szpala spectrometer is about 100 in its valid time range. If the xe2x80x9cghostxe2x80x9d structures are discounted, the signal-to-noise ratio of the Szpala spectrometer is much higher.
The present invention is a positron lifetime spectrometer based on a DC positron beam. It is useful in studies of advanced materials such as characterizing pore structures in thin films. It uses the secondary electrons generated by bombardment of the primary positrons on the sample to start the positron lifetime clock, and uses the detection signal of the annihilation gamma (xcex3) radiation to stop the clock. The spectrometer is of a uniquely simple construction that utilizes the sample potential to simultaneously accelerate both the primary (incident) positrons and the secondary electrons. This construction provides the important benefit of reducing the time spread induced by the energy distribution of the secondary electrons, while also providing a spectrometer operating in a very straightforward manner. In an additional construction feature of the invention, the path of the secondary electrons is separated from that of the incoming positrons. This is achieved by tilting the beam acceleration direction away from the original positron direction, i.e., the direction of the incoming positron beam.
1) D. Schodlbauer, P. Sperr, G. Kogel and W. Triftshauser, xe2x80x9cA Pulsed Positron Beam for Lifetime Studiesxe2x80x9d, Positron Annihilation, eds P. C. Jain, R. M. Singru and K. P. Gopinathan, (World Scientific, Singapore, 1985) p.957-959.
2) R. Suzuki, T. Mikado, H. Ohgaki, M. Chiwaki and T. Yamazaki, xe2x80x9cAn Intense Pulsed Positron Beam and its Applicationsxe2x80x9d, eds E. Ottewitte and A. H. Weiss, AIP Conference Proceedings 303, (AIP Press, New York, 1992) p.526-534.
3) K. G. Lynn, W. E. Frieze and P. J. Schultz, xe2x80x9cMeasurement of the Positron Surface-State Lifetime for A1xe2x80x9d, Phys. Rev. Lett. 52, No. 13, 1137-1140 (1984)
4) S. Szpala, xe2x80x9cDefect Identification Using Analysis of Core-Electrons Contribution to Doppler Broadening of the Positron Annihilation Line and Measurements of Positron Lifetime, Ph.D. Dissertation, The City University of New York, 1999.
A spectrometer for positron lifetime characterization of defects in materials such as thin films, film substrate interfaces, or their substrates is described. A DC positron beam is directed onto a sample surface to produce annihilation gamma radiation and secondary electrons from the sample, the annihilation gamma radiation is detected by a gamma detector, and the secondary electrons are detected by an electron detector. To these common positron lifetime spectrometer components, a single entrance grid is added. The entrance grid is situated in the incident positron beam, is positioned parallel to the sample surface, and is arranged to have a higher electrical potential than the sample potential. In addition, the entrance face of the electron detector assembly is situated parallel to the entrance grid, the entrance face of the electron detector assembly is arranged to have the same potential as the entrance grid, and the sample surface, entrance grid, and entrance face of the electron detector assembly are disposed at a tilt angle to the incident DC positron beam.